Electric fuse



y 2, 1950 A. J. LOWE ETAL 2,506,229

ELECTRIC FUSE Filed March 25, 1949 SULPHUR Jwmow Arthur James Lowe QM Alexander/Yuirhead [[29 Cazdwell W 4 (LAM W Patented May 2, 1950 ELECTRIC FUSE ited,

a corporation of Great Britain Application March 25, 1949, Serial No.

In Great Britain April 30, 1948 2 Claims. (Cl. 102-28) The present invention relates to improvements in electric blasting fuses of the kind especially adapted for shot-firing in a blasting cartridge wherein a gas that does not support combustion is confined under pressure along with a combustible gas-producing heating charge within a pressure resisting container by a sealing device adapted to yield when a predetermined pressure has been built up.

In the "Cardox (registered trade-mark) blasting system which is often employed in fiery or dusty mines, there is employed a pressure-resistant container leading to a vented discharge head with an interposed seal adapted to yield when a predetermined pressure has been built up by the gases produced by the charge in the container, which consists essentially of an ignitable heater charge comprising a mixture of an oxidising agent and one or more organic combustibles, and a charge of carbon dioxide liquefied by loading it under pressure. On ignition of the heater charge, pressure is built up very rapidly under the confinement until the seal yields, whereupon the partly gaseous products of the reaction stream through the discharge head and produce a blasting eifect in the borehole. Owing to the quenchin efiect of the carbon dioxide on the gases from the heater charge, the products emerging from the discharge head do not ignite fiery or dusty atmospheres. The heater charge usually comprises a compact mixture of an inorganic perchlorate and an organic fuel which when ignited in the absence of the liquid carbon dioxide undergoes a strongly exothermic reaction with the production of flame.

It has been proposed in British specification No. 480,330 to provide a blasting cartridge of the kind described containing carbon dioxide or equivalent gas under pressure wherein there is employed a gas-producing charge having a selfcombustible mixture comprising an oxidiser and a fuel in admixture with a non-combustible constituent (which upon combustion of the charge is capable of yielding gaseous products) in a proportion sufflcient to render the charge not selfburning except when confined in the presence of the gas. This minimises the fire risks associated with the manufacturer handling and storage of the heater. It has also been proposed in British specification No. 560,227 to provide a, composition for igniting heating or combustible gas producing charges employed in blasting cartridges in which the blastin medium is a non-combustible gas such as carbon dioxide confined under pressure, and wherein on ignition of the gas producing charge combustion takes place so that heat evolved causes the internal pressure to rise to a point when the cartridge is disrupted or a sealing device is ruptured and the non-combustible gas is suddenly released, this igniting composition consisting of a mixture of an oxidiser and sodium or potassium salicylate, which may if desired contain an endothermic salt in a proportion that renders the composition non selfbuming in air but capable of initiating combustion of the gas-producing charge when under pressure in a cartridge. In accordance with one form of the invention described in this proposal a matchhead for ignitin combustible gas-producing charges of the kind described comprises an electrical resistance wire embedded in a consolidated mixture of 68% potassium perchlorate, 17% sodium or potassium salicylate and 15% ammonium oxalate.

Experimental trials with electric blasting fuses having fuse-head compositions of the aforementioned kind and two diiTering resistance bridgewires of the same metal composition and of linear resistance 1400 ohms per metre and 3500 ohms per metre respectively showed an apparently anomolous behaviour, namely that although both types of fusehead operated the Cardox assembly satisfactorily, having firing currents of 0.50 amp. and 0.30 amp. respectively, nevertheless when tested at atmospheric pressures at a current sufficient to fuse the bridgewires, the fuseheads made with the higher linear resistance wire did not fire in any instance, whereas a large proportion of the fuseheads made with the lower linear resistance wire fired. This led to the belief that whereas the minimum firing current under pressure bore the normal relationship to the linear resistance of the bridgewire, the performance of a fusehead in the atmospheric firing test was related to the total quantity of energy supplied by the bridgewire up to and including the moment of fusion. Therefore it was seen that as the diameter of the bridgewire was increased, ignition at atmospheric frequent.

The object of the present invention is to provide an electric blasting fuse adapted to ignite a heating charge or combustible gas-producing mixture confined under the pressure under which the carbon dioxide or equivalent gas is loaded in a blasting cartridge as aforesaid but which will not fire under atmospheric pressure conditions when an electric current is passed through its resistance bridgewire, and wherein the combustible composition will be free from the corrosive effects associated with the presence of ammonium salts on base metal parts, and espressure would become more pecially brass or other copper metal parts. of the structure in contact with which it is consolidated, whereby its adhesion thereto on long storage is rendered safe.

According to the present invention an electric blasting fuse of the kind especially adapted for shot-firing in a blasting cartridge wherein a gas that does not support combustion is under pressure along with a combustible gasproducing heating charge within a pressure resisting container by a sealing device adapted to yield when a predetermined pressure has been built up comprised a resistance bridge wire of not more than about 1 millimetre length and not more'than 0.025 millimetre thickness and preferably having a specific resistance of at least lOO micro-ohms wholly embedded in an ignitable composition consisting essentially of a mixture of potassium perchlorate and-dinitroso-pentamethylene-tetramine.

It is to be understood that the lower limits of length and thickness of said resistance bridge wire are controlled by those practical constructional considerations well-known in the art of electric blasting fuse manufacture.

Dinitroso pentamethylene tetramine. as is known, may be prepared by a reaction between hexamethylene tetramine and nitrous acid in presence of aqueous nitric acid.

the compositions employed in the construction of the electric blasting fuses according to the invention are capable of ignition under atmospheric pressure conditions when they are sumciently strongly heated, the heat energy that it is permissable to develop in the resistance bridge wire by the passage of an electric current-from the exploder, even if the current used is willcient to fuse it, is limited. For this reason it is essential that the resistance wire should not have a diameter exceeding 0.025 millimetre or a length exceeding about 1 millimetre.

An alloy based on nickel and chromium may be used for the resistance bridge-wire.

While the proportion of potassium perchlorate and 'dinitroso-pentamethylene-tetramine theoretically required to give an equation in which the gaseous oxidation products are carbon dioxide, water and nitrogen is approximately 3:1, a considerable increase of either ingredient can be tolerated. The fusehead-of the electric blasting fuse may conveniently be of the kind in which the pole pieces to which ends of the resistance bridge wire are attached are made of brass foil cemented to an insulating layer, but other forms of construction may be employed e. g. the resistance bridge wire may be attached to the air spaced ends of tinned copper wires kept in parallel relation by means of a plug of insulating material. The composition is conveniently applied in the form of a dope made up with a solution of nitrocellulose or other cement in a volatile solvent from which a bead is built up in known manner. Under atmospheric pressure conditions the resulting electric blasting fuses will not become ignited if an electric current is passed through the resistance bridge wire, e. g. as a result of stray currents, which minimises the hazards of manufacture, handling and storage, but under the high pressure at which the cartridge is charged e. g. atmospheres and over even in the presence of liquid carbon dioxide it becomes ignited and effectively ignites heating charges or combustible gas-producing mixtures such as are used in "Gordon" blasting 4 e. g. those made in accordance with British specification No. 480,330.

The invention is further illustrated in the diagrammatic drawing accompanying the speciflcation, which represents a vertical axial section of electric blasting fuse in accordance with the invention.

In the diagrammatic drawing i is a pressboard sheet, 2 and 2 are brass foils cemented to the press-board sheet I, the foil 2 and a portion of the thickness of the press-board sheet I being cut away at 3 to provide a step bridged by the resistance wire I which is soldered to the foils 2 and 2 at I and 5 8 is a bead of a composition built up by drying of successive coats of a paste of a mixture of potassium perchlorate and dinitroso-pentamethylene-tetramine in the proportions 3:1 by weight in a 5% solution of nitrocellulose in an amyl acetate/amyl alcohol mixture, and subsequently drying of two coats of collodion Ii for waterproofing purposes. I and I are insulated leading wires of which the bated ends are soldered to the foils 2 and 2 at 8 and 8 9 is a paper tube and I0 is a cast sulphur filling from which the bead protrudes, when a current sufllcient to fuse the resistance wire 4 is applied through the loading wires I and 1* while the fuse is exposed at atmospheric pressure, the head 6 falls to ignite, but in a Cardox blasting apparatus in contact with a gas-producing mixture as described in British specification No. 480,330 and charged with carbon dioxide at a pressure of 55 atmospheres the passage of a current even below that recognised to fuse the resistance wire 4 causes the bead l to ignite, and this in turn ignites the gas-producing charge.

After 3 months hot moist storage the fuses still function reliably and the head 6 shows no signs of disintegration or failure to adhere to the brass foils.

We claim:

1. An electric blasting fuse of the kind especially adapted for shot-firing in a blasting cartridge wherein a gas that does not support combustion is confined under pressure along with a combustible gas-produc ng heating charge within a pressure resisting container by a sealing device adapted to yield when a predetermined pressure has been built up, said fuse comprising a resistance bridge wire of not more than about 1 millimetre length and not more 4 than 0.025 millimetre thickness wholly embedded in an ignitable composition consisting of a mixture of potassium perchlorate and dinitroso-pentamethylene-tetram ne.

2. An electric blasting fuse as claimed in claim 1 wherein the resistance bridge wire has a specific. resistance of at least micro-ohms.

ARTHUR J. LOWE.

ALEXANDER MUIRHEAD URE REFERENCES crrEn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,341,263 Burrows Feb. 8, 1944 2,429,490 Scherrer Oct. 21, 1947 2,473,405 Zebree June 14, 1949 2,481,696 Seavey Sept. 13, 1949 OTHER REFERENCES Comptes Rendus, vol. 214, p. 778, 1942. 

